I was at a loss when I was asked recently by my grand-daughter who is a school girl about the difference between Emperor and King. She asked me why Great Britain has King and Queen, while Germany and Russia had the Emperors, and France had both Kings and Emperors. All what I could tell her was, ‘you’d better study by yourself, don’t count on others.”

Being sorry for her, I checked dictionaries at hand, but was simply confounded.

For instance, OALD defines:

Emperor: the ruler of empire.

Empire: a group of countries that are controlled by one ruler / government.

King: the (male) ruler of an independent state / country that has a royal family.

Kingdom: a country ruled by a king (or queen).

The difference seems to be;

King has a royal family.

Empire is a group of countries (under one ruler).

But emperors have royal families as well, and Japan that has had the emperors for a millennium has been a single country (except the imperialism era of 1910–1945), and China who had emperors wasn’t a group of countries.

Can you tell me what is or are the essential (functional and institutional) difference(s) between Emperor and King so that I can give a belated answer to my grand-daughter?

Two comments 1) there might be a definitive distinction, or there may not. The definitions may be vague enough that they could be applied in similar situations, or culturally used even though logically the other may be appropriate. 2) Supposing they had definitive definitions in English as applied to English culture; you're asking how English words might be applied to non-English culture which may are may not have similar combination of issues.
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MitchApr 8 '12 at 14:41

Isn't Akihito as Tennō the ruler of all under the heavens? That would make him bigger than an Emperor :) I suspect that since an Emperor is a ruler of king(dom)s, no other title would have rested well on the brow of the first Japanese monarch (Meiji?) who had a requirement for a western equivalent.
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coleopteristOct 6 '12 at 8:44

11 Answers
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Bringing together a couple of good answers, the primary differences between a King and an Emperor are:

A king rules one "country" or "nation"; an emperor rules over many. This is implicit in the definition of "kingdom" vs "empire"; an empire is always made up of multiple countries that have come under full control of one governing body (typically under one man, sometimes under a small group such as a triumvirate). So, to be an emperor, you have to have an empire; typically you go from "king" to "emperor" by conquering your nearest neighbors.

A king normally rules by birthright; an emperor normally rules by conquest. Note the use of the word "normally". Kingdoms change hands between ruling families; the British Royal Family count ancestors from several formerly-competing houses, meaning that along the way several people crowned king or queen were so crowned after defeating relatives or even entirely different families (in fact perhaps the most famous line of British monarchs, the Tudors, are extinct in the male line; Edward VI, Mary I and Elizabeth I all died childless, and James VI of Scotland, Elizabeth's first cousin twice removed, ascended the throne under the House of Stuart). However, in the normal course of events, the line of succession of rule is by blood; principles of primogeniture apply more often to lines of kings than emperors. Emperors, by the nature of an empire, usually become so by conquest; you start out ruling one country, and then you invade another and replace their former system of government with one controlled by you. The line of succession of emperors can be either by blood (dynastic) or by continued conquest; the Roman Empire is a mix of succession by blood and by usurpment. The key difference is that dynastic rule is usually less important in an empire than simply whomever has the most power and influence within it as of the death or deposition of the sitting Caesar. If the previous Caesar was well-liked, then whomever he has groomed for power (whether a child or a close friend) is likely to ascend. If the previous Caesar was disliked, or didn't leave a clear line of succession, then the situation is ripe for a power grab by a previously less influential faction which then promotes its own leader into the position.

EDIT FROM COMMENT: Actually, the title of Emperor does fit with Japanese history. Prior to the 11th century, Japan was a collection of feudal states basically ruled by the landowners. The Emperor was appointed by these aristocrats to resolve disputes and provide a "unified voice" over these many feudal lands. In the early 11th century, the Kamakura shogunate was formed in the aftermath of the Genpei War, which took governmental power from the aristocrats and Emperor and placed it with the military akin to a junta. During this time, and the following Ashikaga and Tokugawa shogunates, the feudal states on the islands of Japan became unified into what we now know as a single country, and the Emperor's position evolved into more of a spiritual leader akin to the Pope. The importance of bloodline was always central (which does indicate a king), but as the Emperor became the head of the Shinto faith the bloodline came to signify the Emperor's status as a descendant of Amaterasu, and thus having divine legacy but not necessarily the mandate to govern.

In the late 1860s the Tokugawa shogunate fell and power to govern was restored to the Emperor (at the time Emperor Meiji, hence the "Meiji Restoration"). From this time until the end of WWII, Japan did indeed become a true empire, extending beyond the home islands to assert control over Manchuria, most of Southeast Asia and the Indonesian archipelago. The treaty that ended WWII allowed the Japanese people to retain their Emperor as a cultural icon, but stripped the office of all power to govern. The position and its trappings are defined in the current constitution as "a symbol of the Japanese state" akin to the British Crown, but while the Crown retains some key sovereign powers like command of the armed forces, the power to veto and to dissolve Parliament, etc, the Japanese Emperor performs mainly ceremonial and diplomatic duties and has no sovereign power whatsoever.

I appreciate your answer. But ironically it doesn’t seem neither condition -.A king rules one “country” and "nation"; an emperor rules over many, or A king normally rules by birthright; an emperor normally rules by conquest" applies to Japanese emperor. Japan is a single country with no colonies and federal states. Japanese emperors have succeeded sovereign under a single blood line since early 6 century on record. Perhaps Japan’s case is an exception.
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Yoichi Oishi♦Apr 3 '12 at 18:01

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The Pope has never been the Holy Roman Emperor. They are (or were, given one has disappeared) completely different offices.
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MarcinApr 8 '12 at 11:46

You're right. The Papal States, which did fall under the jurisdiction and rule of the Pope, included lands that were also at one time part of the Holy Roman Empire, but I had confused the HRE with the Papal States.
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KeithSApr 9 '12 at 14:24

These terms are also politically loaded (even in ancient cultures). When a ruler wants to seem like a great conqueror whose empire spans vast regions, he will call himself emperor ("I have conquered so many nations, I am a king of kings!") but he can avoid this to emphasize unity ("I may have annexed your country, but we are really the same nation and I have only reunited us - I am still just a king of one people.).
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SuperbestMar 14 '14 at 6:36

King and Emperor are both European titles, with their own (tangled) European history.

King is a Germanic word, derived from the Proto-Indo-European root *genə-, 'birth'. It's related to the English words kin, kind, gentle, etc, as shown in this diagram.

Emperor is a Latin word, derived from the verb imperāre, meaning 'to rule'. The original Latin word was Imperātor, meaning 'ruler'. The term Emperor has been used by many people, in many countries, to describe a ruler who is more powerful than an ordinary King, at least in the opinion of the Emperor, and possibly in the minds of the people he rules.

Essentially, the difference between King and Emperor is like the difference between wonderful and terrific — both are good, and one may be better than the other; but opinion differs about which is which.

George VI of the United Kingdom was both King and Emperor, of course.
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Andrew Leach♦Apr 2 '12 at 22:12

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It is a good learning for me that clear difference is the origin of the word, King from German and Emperor from Latin. Even in the same region, Japan’s sovereign is called the emperor, while that of Thailand is called the king. Louis XVI was king. Napoleon was emperor. The ruler of Saudi Arabia is King, while Haile Shelassie was the last Emperor of Iran. There seems to be no definite rules to decide how to call the ruler other than the ruler himself and his nation.
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Yoichi Oishi♦Apr 2 '12 at 22:55

@Andrew Leach - He was King of England etc but Emperor of India. Emperor because there were many kingdoms in India and therefore many kings. King because there were no other kings in England, Scotland and Wales. The situation in Germany was different. There were many kingdoms, each with a king and one emperor over the whole area. Therefore, I disagree that the difference is only between wonderful and terrific - there is a political aspect too.
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paulApr 3 '12 at 11:53

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@Yoichi: "Haile Shelassie was the last Emperor of Iran" perhaps an error there?
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GEdgarApr 3 '12 at 12:48

It might be helpful to take a corpus-based approach. As a corpus, I have a book from 1904 which reproduces the congratulations sent by heads of state in 1902 to the first President of Cuba. The styles of the various European monarchs challenge some of the suggestions made in other answers in this thread. The letters are written in various languages, but as has been mentioned in other answers there's some common ground between European nations on the terms.

I've skipped one, for brevity and to save transcribing the Cyrillic: Nikolai II of Russia's style can be summarised as: Emperor and Autocrat of all the Russias, of Moscow, Kiev, and a couple of other cities, Tsar of 6 cities/regions/countries, Lord of Pleskov, Grand Duke of five cities/regions/countries, Duke of 11 named places and "others", and we're only half-way through (see note 3).

The first thing which strikes me, and which is consistent with the observation that empires are the result of expansion which engulfs other kingdoms, is that all of the "Emperors" (Wilhelm, Franz Joseph, Edward, Nikolai) also call themselves kings of specific territories. For example, Hungary was part of what we would call the Austro-Hungarian Empire, but its relationship with its ruler was defined in terms of the (historically significant and unique title) Apostolic Majesty rather than the Empire.

The second thing to note is that the four empires were all hereditary, which stands in stark contradiction to some of the assertions made in other answers.

The third point of interest are the principalities. In the case of Bulgaria, which might seem surprising (it's a fairly large place, after all), this is explained by being a vassal of the Ottoman Empire, and it became a Kingdom when it declared its independence in 1908. Denmark is the one which really sparks my curiosity: why did the king style himself a prince?

One common factor of the empires is that they were large and that they included an almagamation of previously independent states. But the same is true of at least two of the kingdoms: Spain (I've seen plenty of references on monuments to Rex Hispaniorum - King of the Spains; and if he wanted to push it then Alfonso could have had a list to rival Nikolai's); and Italy (only recently unified). Both of those kings use a single title in their style, and they're the only monarchs to attribute their rule to something more than the grace of God. The inference can be made that they're intentionally emphasising the unity of their domains and their link with the common people.

In short, an attempt to reduce the distinction between a king and an emperor to a simple criterion seems likely to fail. It seems to be largely a question of historical continuity, politics, and etiquette.

It's an interesting answer and complements the others but not everyone can understand Italian, German, French...
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Mari-Lou ASep 9 '13 at 4:41

@Mari-LouA, this is true, but I'm not sure how to add glosses without distracting from the flow. Do you think it would be useful to bold the words which mean emperor, king, prince, lord, or regent?
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Peter TaylorSep 9 '13 at 7:35

Maybe add the English translation in italics next to each title of aristocracy? E.g., Re d'Italia (King of Italy)?
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Mari-Lou ASep 9 '13 at 11:04

If the ruler of Denmark was a Regent, by definition he was not a king; Prince Christian sent congratulations in the name of the King (notice significant comma). Also, translating Kaiser 'Emperor' is not entirely straightforward (Caesar is not Imperator).
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TimLymingtonSep 9 '13 at 17:42

@TimLymington, Christian IXwas the king of Denmark. Kaiser is discussed more elsewhere on this page; while Caesar and Imperator are technically distinct, many Roman emperors after the original Julio-Claudian dynasty chose to style themselves Caesar.
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Peter TaylorSep 9 '13 at 18:10

This is not true. China and Japan were not united under an emperor, unless you're considering the brief period from ~1935-1945 when Japan conquered portions of China and brought them under the dominion of the Japanese emperor. And a king may call himself an emperor, but others would not unless he had dominion over multiple kingdoms.
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RobustoApr 8 '12 at 12:56

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@Robusto - Jumping in a bit late... I don't think Bhwong was trying to say that they were united under one Emperor, but that each of them was separately united by an emperor. Instead of China and Japan was not a country, I suspect that Neither China nor Japan was a single country is what was intended.
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MT_HeadMay 11 '12 at 17:04

You may be interested in the concept of Divine Right, which (to me, as a citizen of a monarchy) implies that the King is there to be complementary to and wholly part of the nation, and vice versa. It suggests that the natural state of being for a country is to have a King, as specified by God. This presumption does not necessarily apply to Emperors, whom are acknowledged to come to power by the machinery of politics, including birthright, or conquest.

In other words, whether or not this idea is accepted academically, it seems to me that the idea of a King is more closely tied to the concept of nationhood than that of an Emperor.

(I am not entirely happy with my terminology, as I think the concept of 'nationhood' is probably newer than that of Kings, and encompasses a more complete idea of politics and representation compared to the realm that Kings are supposed to preside over in the concept of Divine Right. In this respect, since rulers of self-defined Nations have called themselves Emperor, I need an older word for the domain of a King.)

Historically, the main difference between "king" and "emperor" might have been how they came into office.

A king usually becomes king because his family has been the ruling family, and he ascends to the throne because he is next in line after the old king (who died). Usurpers are an exception but would call themselves king because they would found a new dynasty. (Similarly for queen, of course.)

An emperor in old Rome, where the term arguably originated, that is an "imperator" was a successful general. He would carry the title from victory until his glorious return to Rome.

Dictator was an office that the Roman senate would staff with a suitable candidate in times of (military) need; this was similar to modern martial law. The mandate would end after a predetermined period of time (if not extended).

Caesar is a title (which is the origin of German "Kaiser") which was used in its modern meaning only after Gaius Iulius Caesar (where Caesar is only a name), who was dictator first, (illegaly) became sole ruler of the Roman Empire, beginning the era of Caesars.
In the Holy Roman Empire the "Kaiser" was elected by the principle dukes ("Kurfürsten") and arch bishops (and then appointed by the (a, at times) pope).

In modern English, the terms "emperor" and "caesar/Kaiser" have somehow merged. The meaning of "dictator" shifted considerably (arguably due to office abuse in the past). The main difference (historically) between those and kings seems to be that the former are granted (or earned) titles while kingship is inherited.

Of course, tyrants declare themselves anything they want so there have certainly been other constellations.

I would add to the other excellent answers that, at lease to my mind, King implies, as noted by @JohnLawler, birth. Ruling by bloodline. Whereas an Emperor generally seems to have risen to station via political or military might. Even though they may have started life as a King, they can upgrade to Emperor by taking over other countries, as in the case of England and India.

There's probably a lot of reverse engineering here as well, but I assume that a duchy and a principality are synonymous with dukedoms and princedoms
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tinydApr 3 '12 at 15:03

I think that depends on what time period and which country. These two are not synonyms in Britain and this is a Q&A about english IIUC.
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David Allan FinchApr 3 '12 at 15:27

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@David: English, but not necessarily England. You and I know that the United Kingdom is the only one that matters, but not everybody does :)
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TimLymingtonApr 3 '12 at 17:10

@tinyd: Cornwall and Wales are a duchy and a principality respectively, while the Dukedom of Cornwall and Princedom(?) of Wales are titles that were bestowed on Prince Charles.
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TimLymingtonApr 3 '12 at 17:13

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@TimLymington - Aha, so duchy and principality are used to refer to the place, while the '-dom' versions are used when referring to the title. Thanks for the clarification.
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tinydApr 4 '12 at 6:53

I wouldn't try to attach precise definitions to such "job titles". The president of France has very different powers than the president of the United States. An American consul is a very different job from the ancient Roman consul. For that matter, the powers of the Queen of England today have little relation to the powers of the Queen of England when Victoria or Elizabeth I held the office.

This is doubly so when the title is a translation from another language.

Under the Constitution, Elizabeth II has very similar powers to Victoria, although the character of the Commonwealth and being Head of State of only 16 Realms is very different to Victoria's status as monarch of the British Empire and Empress of India.
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Andrew Leach♦Apr 3 '12 at 8:17

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This is true, but it is difficult to see how she could use these powers without the consent of parliament. Parliament has twice now removed a King, I doubt without a popular mandate any Queen or King would try again. In a sense the People and Parliament are now Sovereign in the Uk because they in reality (if not name) have the power. The powers that she does have she uses with consent of the Prime Minister. This is kind of a double lock on these powers which was the Great Comprise.
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David Allan FinchApr 3 '12 at 8:38

I was under the impression that Britain doesn't have a Constitution in the sense of a single written document that defines the powers of the various parts of the government. In any case, I think if the queen of England today were to, say, declare war, or order the execution of a political opponent, we would quickly see that she does not have the same powers as monarchs past.
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JayApr 4 '12 at 14:59

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Britain doesn't have a Constitution in the same way as the United States. But the powers of the Crown are set out in various documents like Magna Carta, the Bill of Rights and the some Acts of Parliament. I'm fairly certain that it is only the monarch who can declare war, but it's only done on the advice of ministers (so if they say Yes, then it is war; if they say No, it isn't). That's the same as Victoria, but not the same as Elizabeth I.
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Andrew Leach♦Apr 4 '12 at 22:40

To the linguistic changes, mentioned by John Lawler, I have to add small detail: both King and Emperor (Kingdom and Empire) have full sense only inside Europe. For other regions they are only more or less correct translation.

You know better than others, that "Emperor of Japan" is european-style title of Tennō, which can be dirty translated into English as "heavenly sovereign". Same details applied also to China-Iran-...

And I want to fix OALED (what is it?) definition. Emperor can rule both: a group of formally independent countries (Napoleon) and a multinational unitary state, created from independent in past countries (or pre-state entities). Formally empires have more territory, more population, a more diverse national structure without overwhelming numerical dominance of individual nation

1.We call our emperor “Tenno,” of which verbatim translation is “Heavenly king” as you say. We had the Constitution of literally and officially “The Great Japan Empire – 大日本帝国憲法” from 1889 through 1947, which was replaced with the current “The constitution of Japan – 日本国憲法.” after the World War II. In English, I think both Tennno Hirohito and Akihito are called Emperor Hirohito and Emperor Akihito, and not King Hirohito and Akihito, or Sovereign Hirohito and Akihito. 2. I abbreviated Oxford Advanced Learners’ English Dictionary as OALED.
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Yoichi Oishi♦Apr 3 '12 at 4:01

@YoichiOishi - if you had Empire (and you had it from 8/?/ century), Emperor rule it, no doubt
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Lazy BadgerApr 3 '12 at 15:49

Japan called it “the great empire” by herself from the time Emperor Mutsuhito, a.k.a. Meiji Emperor proclaimed “the Great Japan Empire Constitution” in 1892 until the defeat in the World War II (1945). During the time of 1910 through 1945, Japan’s sovereignty and influence covered Korea, Taiwan, half of Sakhalin, Manchuria (Northern east part of China), a part of the Kuril Islands and some islands in the South Pacific on top of the present territory of Japan. I would call it the time we formed the empire.
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Yoichi Oishi♦Apr 3 '12 at 23:40

The difference you mentioned is correct. It's important to note, however, that an emperor doesn't necessarily have to be an emperor of countries, in the modern sense. I believe, in the examples of Japan and China, the empires arose when the territory we now refer to as countries was actually a large collection of independent city-states and other small sovereign bodies. The empires arose to impose some degree of universal authority over all those sovereignties, but left some amount of autonomy and private governance in place.

Both imperial and monarchial families can and have been considered royalty or more (for example, many empires have regarded their emperors as deities.

This applies to Germany too. Lots of independent duchies were eventually ruled by a Kaiser (Emperor).
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Andrew Leach♦Apr 2 '12 at 22:14

Monarchical or (rarely) monarchial refers to the form of government; the family of the monarch is the Royal Family. There can be nothing 'more' than that; Debrett's once ruled that "The many millions of followers of His Highness the Aga Khan claim that he is directly descended from God - but an English Duke takes precedence."
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TimLymingtonApr 2 '12 at 22:21

@AndrewLeach - Kaiser derives from the Latin Ceasar - the surname many of the later Roman Emperors used to try to give themselves a bit of the luster of Julius Ceasar. The title in Germany dates back to the Holy Roman Empire, which got the name "Empire" because it originally included large parts of Italy as well as Germany. After German unification the Prussian rulers took up the title "Kaiser" again, but it was probably more aspirational than anything else. They ruled over a large number of Poles, did briefly take a few small Pacific islands, and put in a claim for a slice of Africa.
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T.E.D.Sep 9 '13 at 15:15